Amplitude control device for electrical oscillator and electrical oscillator comprising such a device

ABSTRACT

An amplitude control device for a signal output by an oscillator includes a rectification circuit for rectifying the output signal, and a differential amplification circuit for generating a biasing current control signal for the oscillator. The biasing current control signal is based upon the output signal from the rectification circuit and a reference voltage. A dividing bridge and an adder are designed so that only a fraction of the reference voltage is used to define the amplitude of the oscillations. The contribution made to the oscillator phase noise by the reference voltage noise is considerably reduced.

FIELD OF THE INVENTION

[0001] The present invention relates to an amplitude control device foran electrical oscillator, and an electrical oscillator comprising such adevice.

[0002] The present invention is applicable to radio communicationsystems, and more particularly, to mobile telephone systems, such asGSM, DCS, PCS, UMTS, etc.

BACKGROUND OF THE INVENTION

[0003] An electrical oscillator is a circuit that produces an electricalsignal at a frequency defined by a time constant specific to it. FIG. 1illustrates a principle diagram for an electrical oscillator thatincludes an inductance L, a capacitor C and a holding amplifier A. Theoscillator time constant is equal to {square root}{square root over(LC)} and the oscillation frequency is equal to {fraction (1/2)}π{squareroot}{square root over (LC)}.

[0004] The amplitude of voltage oscillations measured at the terminalsof the LC circuit is determined by non-linearities of the holdingamplifier. It is important to check the amplitude of the voltageoscillations to achieve correct interfacing between the oscillator andthe oscillator load circuits that receive the oscillation voltage ontheir inputs.

[0005] If the oscillator is integrated on silicon, manufacturingparameter dispersions affect the value of the amplitude of oscillations.For example, this is the case for the resistivity of metals for whichthe dispersions (+/−10%) modify the quality coefficient of theinductances and the capacitors, or the resistivity of poly-crystallinesilicon for which the dispersions (+/−20%) modify the oscillator biasingcurrent.

[0006] One known way of reducing these dispersions is to make amplitudeslaving circuits that apply a retroaction on the oscillator biasingcurrent. For example, this type of slaving circuit is described in thearticle titled “A 2V 2.5 GHz-104 dB/Hz At 100 kHz Fully Integrated VCOWideband Low Noise Automatic Amplitude Control” (Alfio Zanchi et al,IEEE JSSC, VOL 36, No. 4, pp. 611-619, April 2001), and in the articletitled “A Low Noise Low Power VCO With Automatic Amplitude Control ForWireless Application” (M. A. Margarit et al., IEEE JSSC, VOL 34, No. 6,pp. 761-771, June 1999).

[0007] An example of an oscillator with its slaving circuit according toknown art is shown in FIG. 2. The circuit in FIG. 2 comprises anoscillator 1, a rectification stage 2 and a differential amplifier 3.The oscillator 1 comprises two transistors Q1, Q2, two resistances R1,R2, two inductances L1, L2, three capacitors C1, C2 and C3, and acurrent generator C. Three biasing voltages VBB, VCC, VEE power theoscillator 1. The output voltage from the oscillator 1 is taken from theterminals of capacitor C1.

[0008] The rectification stage 2 receives the output voltage from theoscillator on its input and detects the peak level of the oscillationvoltage, for example, by double alternation rectification. The outputvoltage from the rectification stage is applied to a first input to thedifferential amplifier 3. A reference voltage Vref is applied to thesecond input of the differential amplifier 3. The output signal from thedifferential amplifier 3 controls the amplitude of the biasing currentthat passes through the current generator G. Thus, the amplitude of theoutput voltage from the oscillator 1 is controlled by the level of thereference voltage Vref.

[0009] This type of circuit has the disadvantage that it copies all thenoise of the reference voltage Vref for frequencies within the slavingpassband, and that there is no noise control outside this passband. Thiscopying affects the phase noise in addition to the amplitude noise.Ideally, amplitude regulation should only generate an amplitudemodulation. However, the non-linear nature and parametric effects of theoscillator cause the amplitude noise to be converted to phase noise,thus deteriorating the spectral quality of the oscillation signal.

SUMMARY OF THE INVENTION

[0010] The invention relates to an amplitude control device for a signaloutput by an oscillator. The amplitude control device comprises slavingmeans comprising rectification means to rectify the signal output by theoscillator, and differential amplification means to form a controlsignal for the oscillator biasing current starting from the signal takenat the output from the rectification means and a reference voltage.

[0011] The slaving means further includes a divider bridge to form afirst fraction of the reference voltage and a second fraction of thereference voltage starting from the reference voltage. The firstfraction of the reference voltage is applied to a first input of thedifferential amplification means. An adder adds the signal output by therectification means and the second fraction of the reference voltage.The signal output by the adder is applied to a second input to thedifferential amplification means.

[0012] Advantageously, the invention considerably reduces thecontribution of the reference voltage noise to the oscillator phasenoise. Only a fraction of the reference voltage is used to define theamplitude of the oscillations. Consequently, for frequencies less thanthe amplitude slaving passband, the noise injected into the oscillatoris proportional to the product of this fraction by the reference voltagenoise.

[0013] According to one improvement to the invention, the devicecontrolling the amplitude of the signal output by an oscillatorcomprises additional slaving means to control the oscillator noise levelfor frequencies greater than frequencies within the passband of theslaving means.

[0014] The invention also relates to an electrical oscillatorcharacterized in that it comprises a signal amplitude control deviceaccording to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Other characteristics and advantages of the invention will becomeclear after reading a preferred embodiment of the invention withreference to the attached figures among which:

[0016]FIG. 1 is a schematic diagram of an electrical oscillatoraccording to the prior art;

[0017]FIG. 2 is a schematic diagram of an electrical oscillator with anoscillation amplitude slaving circuit according to the prior art;

[0018]FIG. 3 is a schematic diagram of an electrical oscillator with anoscillation amplitude slaving circuit according to the presentinvention;

[0019]FIG. 4 is a more detailed schematic diagram of the electricaloscillator shown in FIG. 3;

[0020]FIG. 5 is a schematic diagram of an electrical oscillator with animprovement to the circuit shown in FIG. 3; and

[0021]FIG. 6 is a more detailed schematic diagram of the electricaloscillator shown in FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The same references denote the same elements on all figures.FIGS. 1 and 2 have already been described, therefore these figures willnot be described again. FIG. 3 shows an electrical oscillator with anamplitude slaving circuit according to the invention. The circuit inFIG. 3 comprises an oscillator 1 and an amplitude slaving circuit 4. Theoscillator 1 is identical to the oscillator shown in FIG. 2. Theamplitude slaving circuit 4 comprises a voltage divider bridge 5, arectification stage 6, an adder 7 and a differential amplifier 8.

[0023] The input terminals to the amplitude slaving circuit 4 are theinput terminals to the rectification stage 6. The amplitude slavingcircuit 4 output is the output from the differential amplifier 8. Afirst capacitor C4 is between the first output terminal of theoscillator 1 and a first input terminal to the rectification stage 6.Similarly, a second capacitor C5 is between a second output terminal ofthe oscillator 1 and a second output terminal from oscillator 1.

[0024] The voltage divider bridge 5 comprises three resistances R8, R9,R10 mounted in series and a decoupling capacitor C9 is mounted inparallel with resistances R9 and R10. The reference voltage Vref isapplied to a first terminal of the resistance R8, the second terminal ofwhich is connected to a first input (+) of the differential amplifier 8and to the first terminal of the resistance R9 The second terminal ofthe resistance R9 is connected to a first input of adder 7 and to afirst terminal of resistance R10. The second terminal of resistance R10is connected to a power supply voltage VEE.

[0025] The output from the rectification stage 6 is connected to asecond input of an adder 7, the output of which is connected to thesecond input (−) of the differential amplifier 8. The output signal fromthe differential amplifier 8 forms the amplitude control signal for thebiasing the current that passes through the current generator G.

[0026]FIG. 4 illustrates an example electrical oscillator like thatshown in FIG. 3. Apart from the divider bridge 5, the amplitude slavingcircuit 4 comprises two P-type MOS (Metal Oxide Semiconductor)transistors MP1 and MP2, three bipolar transistors Q4, Q5 and Q6, twodecoupling capacitors C7, C8 and three resistances R5, R6 and R7. Thetransistors Q4 and Q5 perform the rectification. The differentialamplifier 8 is composed of transistors MP1, MP2, Q4, Q5, Q6 and theresistance R7. The participation of transistors Q4 and Q5 in therectification and differential amplification functions has the advantageof limiting the number of stages in the amplitude slaving circuit 4, andconsequently, obtaining better noise performances of this circuit.

[0027] Transistors Q4 and Q5 are mounted on a common emitter. The commonemitter of transistors Q4 and Q5 is connected to a first terminal of theresistance R7, the second terminal of which is connected to a powersupply voltage VEE. The decoupling capacitor C8 is parallel with theresistance R7.

[0028] The bases of transistors Q4 and Q5 form the inputs to therectification circuit 6 and are connected to a first terminal of theresistance R5, and to a first terminal of resistance R6. The secondterminals of the resistances R5 and R6 are both connected to a firstterminal of capacitor C7. The second terminal of the capacitor C7 isconnected to the power supply voltage VEE.

[0029] The collectors of transistors Q4 and Q5 are connected to eachother and form the output from the differential amplifier. Thecollectors of Q4 and Q5 are connected to the source of the transistorMP1, and the drain of this transistor is connected to the biasingvoltage VCC. The collector of transistor Q6 is connected to the sourceof transistor MP2, the drain of which is connected to the biasingvoltage VCC. The gates of transistors MP1 and MP2 are connected to eachother and to the source of transistor MP2. The emitter of transistor Q6is connected to the emitters of transistors Q4 and Q5. The base oftransistor Q6 is connected to the second terminal of resistance R8. Thesecond terminal of resistance R9 is connected to the second terminals ofresistances R5 and R6.

[0030] The current generator G includes a resistance R4 in series with aN-type MOS transistor MN1. The source terminal of the transistor MN1 isconnected to the DC voltage VEE. The output voltage from the slavingcircuit, which is picked up on the collector common to transistors Q4and Q5, is applied to the gate of transistor MN1. A capacitor C6 isbetween the grid of transistor MN1 and the voltage VEE.

[0031] In the remainder of the description, the voltages reference U1,U2, U3, U4, U5 and U6 represent the following potential differences: thepotential difference taken between the node common to resistances R8 andR9 and the circuit ground; the potential difference taken between thenode common to resistances R9 and R10 and the circuit ground; thepotential difference taken between the emitters common to transistorsQ4, Q5 and Q6 and the circuit ground; the potential difference takenbetween the first input to the slaving circuit 4 and the circuit ground;the potential difference taken between the second input to the slavingcircuit 4 and the circuit ground; and the potential difference takenbetween the output from the slaving circuit 4 and the circuit ground.

[0032] When the oscillator has not yet started, the voltages U4 and U5are equal to U2 and the differential amplifier composed of Q4, Q5, Q6,R7, MP1 and MP2 is unbalanced since the voltage U1 is greater than U4and U5. Consequently, the voltage U6 is close to VCC. The transistor MN1then has a minimum resistance Ron (MN1) which creates a maximum biasingcurrent for the oscillator so that oscillations can start.

[0033] The amplitude of the oscillations then increases gradually, thussuperposing two alternating voltages with opposite phases onto the DCcomponent of the voltages U4 and U5 such that when a positivehalf-alternation of the voltage U4 takes place, and transistor Q5 isblocked, transistor Q4 outputs current Ie(Q4) such that:${{Ie}({Q4})} = {{Is} \cdot e \cdot \frac{{U4} - {U3}}{P}}$

[0034] where P is the thermodynamic potential.

[0035] The situation is reversed during a positive half-alternation ofU5. The currents that pass through the emitters of transistors Q4 and Q5are summed and filtered by the capacitor C8, and the voltage U3 is animage of the average value of these currents. When this average value ofthe current is equal to the DC current output by the transistor Q6, thefollowing is true: $\begin{matrix}{{< {{Ie}({Q4})}>={\frac{2}{T}{\int_{0}^{\frac{T}{2}}{{{Is} \cdot e}\frac{{U4} - {U3}}{P}\quad {t}}}}},} \\{{{{Ie}({Q6})} = {{{Is} \cdot e}\frac{{U1} - {U3}}{P}}},}\end{matrix}$

[0036] and

Ie(Q 6)=<Ie(Q 4)>

[0037] where p is the thermodynamic potential, <Ie(Q4)>is the averagevalue of the current Ie(Q4), and T is the period of the oscillationsignal.

[0038] Also:${{U4} + {U2} + {{Um} \cdot {\sin \left( {2 \cdot \pi \cdot \frac{t}{T}} \right)}}},$

[0039] where Um is the amplitude of the signal present on one of theoscillator output nodes If the filtering done by the capacitor C8 issufficiently efficient, it can be assumed that the voltage U3 isconstant and we then have: $\begin{matrix}{{\frac{2}{T}{\int_{0}^{\frac{T}{2}}{{{Is} \cdot ^{\frac{{U2} + {{Um} \cdot {\sin {(\frac{2 \cdot \pi \cdot t}{T})}}}}{P}}}\quad {t}}}} = {{Is} \cdot e \cdot \frac{U1}{P}}} \\{{{namely}:\quad {\frac{2}{T}{\int_{0}^{\frac{T}{2}}{^{\frac{{Um} \cdot {\sin {(\frac{2 \cdot \pi \cdot t}{T})}}}{P}}{t}}}}} = ^{\frac{{U1} - {U2}}{P}}}\end{matrix}$

[0040] if we make a variable substitution $\begin{matrix}{{x = \frac{2 \cdot \pi \cdot t}{T}},{{{we}\quad {{obtain}:\quad {\ln \left\lbrack {\int_{0}^{\pi}{^{\frac{{Um} \cdot {\sin {(x)}}}{P}}\quad {x}}} \right\rbrack}}} = \frac{{U1} - {U2}}{P}}} & \lbrack 3\rbrack\end{matrix}$

[0041] Equation 3 shows that the amplitude Um only depends on thedifference (U1−U2) that is a fraction of the reference voltage Vref. Wehave:$\left( {{{U1} - {U2}} = {\frac{R9}{{R8} + {R9} + {R10}}\quad {VREF}}} \right)$

[0042] Thus, the noise of the reference voltage Vref injected into theslaving is also multiplied by only this fraction of the referencevoltage, which reduces its contribution to the oscillator phase noise.

[0043] It may be noted that the circuit according to the invention canbe used to adapt the noise quantity injected by the reference voltage tothe amplitude of the oscillations. Equation [3] shows that a reductionin the reference voltage Vref causes a reduction in the quantity U1−U2,and consequently a reduction in the amplitude Um of the oscillations.The oscillators thus made are only slightly sensitive to the referencevoltage noise.

[0044] However, for frequencies greater than the frequencies of thepassband of the slaving circuit 4, there is some phase noise thatreturns from the oscillator. This noise return phenomenon isparticularly visible if the transistor MN1 is operating in its saturatedzone. It is then necessary to guarantee that MN1 operates in its pureresistance zone. This condition is satisfied by the use of an additionalslaving circuit.

[0045]FIG. 5 shows a slaving device comprising such an additionalslaving circuit. FIG. 6 shows an example embodiment of the slavingdevice shown in FIG. 5. With reference to FIG. 5, the additional slavingcircuit 9 controls the oscillator noise level for frequencies greaterthan the frequencies within the passband of the slaving circuit 4.

[0046] The additional slaving circuit 9 is composed of a differentialamplifier, a first input of which is connected to the emitters oftransistors Q1 and Q2, and a second input of which is connected to a DCvoltage VXX. The output of the differential amplifier is connected tothe terminal common to resistances R1 and R2 of the oscillator 1, whichis therefore no longer connected to a DC voltage VBB as is the caseaccording to the prior art.

[0047] According to the example shown in FIG. 6, the circuit 9 comprisesa resistance R3 and a transistor Q3. The first input, the second inputand the output from the differential amplifier respectively correspondto the base, the emitter and the collector of transistor Q3. The emitterof transistor Q3 is connected to the power supply voltage VEE (and thenVXX=VEE), and the collector of transistor Q3 is connected to a firstterminal of the resistance R3, the second terminal of which is connectedto the power supply voltage VCC. The resistance R3 fixes the biasingcurrent of the second slaving. The resistance R4 of the currentgenerator is chosen such that the transistor MN1 operates in its pureresistance zone. The following relation must then be satisfied:

Vbe(Q 3)−R 4.Iosc>Vdss(MN 1), where

[0048] Vbe(Q3), Iosc and Vdss(MN1) respectively represent the voltage onthe base of transistor Q3, the oscillator biasing current, and thedrain/source saturation voltage of transistor MN1.

[0049] It may also be noted that the additional slaving circuitcontributes to reducing the phase noise of R1, R2, Q1 and Q2 forfrequencies less than its passband. This statement is particularlyrelevant if a CMOS type technology is used (HCMOS9 type), if transistorsQ1 and Q2 are replaced by NMOS transistors, and if Q3 is a native NPNtransistor based on this technology. Under these conditions, thecontribution of low frequency noise from NMOS transistors to theoscillator phase noise is almost entirely canceled.

[0050] This invention is applicable to oscillators in general, andparticularly to voltage controlled oscillators (VCO).

That which is claimed is:
 1. Amplitude control device for a signaloutput by an oscillator, the device comprising slaving means (4)composed of: rectification means (6) to rectify the signal output by theoscillator, differential amplification means (8) to form a controlsignal for the oscillator polarization current starting from the signaltaken from the output from the rectification means (6) and a referencevoltage (Vref), a divider bridge (5) to form a first fraction of thereference voltage (U1) and a second fraction of the reference voltage(U2) starting from the reference voltage (Vref), the first fraction ofthe reference voltage (U1) being applied to a first input to thedifferential amplification means (8), and an adder (7) to add the signaloutput by the rectification means (6) and the second fraction of thereference voltage (U2), the signal output by the adder (7) being appliedto a second input to the differential amplification means (8),characterized in that: the voltage divider bridge (5) comprises threeresistances R8, R9, R10 mounted in series, the reference voltage Vrefbeing applied to a first terminal of resistance R8, the second terminalof which is connected to a first terminal of resistance R9, the secondterminal of which is connected to a first terminal of resistance R10,the second terminal of which is connected to a power supply voltage VEE,the first fraction of the reference voltage (U1) being taken from thesecond terminal of the resistance R8 and the second fraction of thereference voltage (U2) being taken from the second terminal of theresistance R9, and in that the rectification means (6), the differentialamplification means (8) and the adder (7) comprise two P type MOStransistors, MP1 and MP2, three bipolar transistors Q4, Q5 and Q6, twodecoupling capacitors C7, C8 and three resistances R5, R6 and R7, thetransistors Q4 and Q5 being mounted with a common emitter, the commonemitter of Q4 and Q5 being connected to a first terminal of resistanceR7, the second terminal of which is connected to a power supply voltageVEE, the decoupling capacitor C8 being mounted in parallel withresistance R7, the bases of transistors Q4 and Q5 forming the inputs tothe rectification circuit 6 and being connected respectively to a firstterminal of resistance R5, the second terminal of which is connected toa first terminal of capacitor C7, and to a first terminal of resistanceR6, the second terminal of which is also connected to the first terminalof capacitor C7, the second terminal of capacitor C7 being connected tothe power supply voltage VEE, the collectors of transistors Q4 and Q5being connected to each other and forming the output from thedifferential amplifier (8), the collectors of Q4 and Q5 being connectedto the source of transistor MP1, the drain of which is connected to apolarization voltage VCC, the collector of transistor Q6 being connectedto the source of transistor MP2, the drain of which is connected to thepolarization voltage VCC, the grids of transistors MP1 and MP2 beingconnected to each other and to the source of transistor MP2, the emitterof transistor Q6 being connected to the emitters of the transistors Q4and Q5, the base of transistor Q6 being connected to the second terminalof resistance R8, the second terminal of resistance R9 being connectedto the second terminals of resistances R5 and R6.
 2. Device according toclaim 1, characterized in that it comprises additional slaving means (9)to control the oscillator noise level for frequencies greater thanfrequencies within the passband of the slaving means (4).
 3. Deviceaccording to claim 2, characterized in that the oscillator polarizationcurrent is fixed by a current generator (G) composed of a resistance(R4) in series with a transistor (MN1), the value of the resistance (R4)of the current generator (G) being chosen such that the transistor (MN1)works in its pure resistance zone.
 4. Electrical oscillator,characterized in that it comprises a signal amplitude control deviceaccording to any one of the previous claims.